You spend lots and lots of money on dedicated hardware, but this is bound
to get you talked about.

Seriously, you can't if they are Unix password files - the Unix password
system employs one-way encryption. It's more like hashing than encryption.
The best you can check is whether something else hashes to the same string.
You can't turn a hash back into the original string. Programs like Crack
can forcibly (and intelligently) try to guess passwords, but don't (can't)
guarantee quick success.

If you're worried about users selecting bad passwords, you should proactively check when they try to change their password (by modifying
passwd(1), for example).